1. Technical Field
The present invention relates to a color filter substrate and a liquid crystal display (LCD) device using it, and in particular, relates to the color filter substrate without an overcoat layer and the LCD device with it.
2. Background Art
In recent years, a flat panel display device such as the LCD device is widely used for its features of thinness, light-weight and low power consumption. Conventionally, such flat panel display devices have been mainly used as monitoring display for desktop computers and laptop computers. Now, the flat panel display devices are used for TV, and thus a more vibrant display is requested, that is, a display with the high color reproducibility is highly requested. In order to achieve a color display, many flat panel display devices are generally equipped with a color filter in which the color layer patterns are arranged so as to correspond to pixels. The above-mentioned color reproducibility is improved by adjusting optical spectrum of a light source of the flat panel display device and the optical spectrum of the color filter. In the LCD device, spectral adjustment for a light source such as three wavelength type cold cathode fluorescent lamp (CCFL) backlight and a light emitting diode (LED) backlight are performed and the spectral adjustment for the color layer of the color filter substrate used for a LCD panel is performed.
As for production of the color filter substrate, in general, a printing process, a photoresist method or an etching method is used. Among those methods, however, when considering a high-resolution controllability of the spectral properties and reproducibility, the photoresist method is most desirable. The photoresist method is a method to make a color filter by repeating color forming steps, which forms each color layer by performing pattern exposure and development, after coating a film of a coloring composition on the transparent substrate. The coloring composition is made by dispersing a pigment into a transparent resin together with a suitable soluble agent with photoinitiator and the polymerization monomer. The color filter is checked for a quality assessment by checking a film thickness unevenness at the time of coating, an irregular color due to aggregation of the color resist and pattern defects.
Cost reduction for the flat panel is also attempted in the TV use. Therefore, in consideration of cost reduction of various members composing the flat panel, its fundamental structure is reconsidered. In the LCD device for example, by reconsidering the structure of the color filter substrate as disclosed in the patent document 1 (Japanese Patent Application Laid-Open No. 1994-337308), the number of process and the material cost are reduced. The patent document 2 (Japanese Patent Application Laid-Open No. 1997-211441) discloses a configuration using black resin material instead of conventional chromium metal as the material of the black matrix (hereinafter, referred to as a BM) layer.
Here, a conventional structure of the color filter substrate and manufacturing method will be described with reference to drawings. FIG. 15 shows an example of a general color filter substrate coated with an overcoat layer (hereinafter, referred to as an overcoated color filter substrate). Each of FIG. 16 and FIG. 17 shows a cross sectional view along I-I line and II-II line of FIG. 15, respectively. FIG. 18 shows an example of the color filter substrate without the overcoat layer (hereinafter, referred to as a non-overcoated color filter substrate) using the BM layer of a resin material. Each of FIG. 19 and FIG. 20 shows a cross sectional view along I-I line and II-II line of FIG. 18, respectively. FIG. 21 is a cross sectional view showing the color filter substrate disclosed in patent document 2.
A process of manufacturing the non-overcoated color filter substrate 11 using the resin BM layer shown in FIGS. 18 to 20 will be described. First, a resin BM resist is applied on a glass substrate 12, and then exposed, developed and baked, and a first BM pattern (a resin BM pattern 13 such as a mesh pattern or a lattice pattern for defining pixels) in the display area 150 and a second BM pattern (a resin BM frame 14) is formed so as to be located on a peripheral area of the display area. Next, a color resist is applied, exposed, developed and baked, and the color layer pattern (a red color layer 151, a green color layer 152 and a blue color layer 153) is formed. After that, the surface polishing is performed if needed, and an alignment film 17 is formed, and then columnar spacers are provided to form a cell gap so as to be filled with liquid crystal.
In case of the overcoated color filter substrate 11 using the resin BM layer shown in FIG. 15 through FIG. 17, such as the color filter substrate for IPS (In Plane Switching) modes, for example, additional processing is included as disclosed hereinafter. In a manufacturing method of the above-mentioned overcoated color filter substrate, after a color layer pattern formation process, an overcoat layer formation process is inevitable wherein an acrylic resin or an epoxy resin is coated on the BM pattern and the color layer pattern and baking process is performed.
And then alignment process is performed on both color filter substrate 11 and an opposed substrate such as a thin film transistor (TFT) substrate which is provided with switching elements of TFT or the like arranged by a matrix pattern. After that, a liquid crystal (LC) panel is formed by either a liquid crystal dropping method or a liquid crystal injection method. After that, a pair of polarizing plates is provided on the LCD panel. The above-mentioned columnar spacers would be replaced by spherical spacers to form the cell gap of the LC panel.
In the color filter substrate which achieved cost reduction by not using the overcoat layer, various display defects tends to occur because the flatness of the surface of substrate is insufficient. Specifically, although the overcoat layer includes the function to mainly prevent dissolution of impurities from the color layer to the liquid crystal and the function to reduce a step of the surface of the color filter substrate, when the overcoat layer is eliminated, defects related to flatness deterioration occur. For example, halftone display blur due to uneven rubbing process or a contrast decline owing to light leak during black display are tended to occur. When resin material is used as the BM layer, those litter produced from the alignment film are observed at a step portion of the resin BM layer, e.g., stripped portions of the alignment film and fixed waste of the alignment film at the time of rubbing the alignment film. Those litter moves in the display area during a vibration test, and generates a defect called “vibration spot” and deteriorates the display quality.
In order to make the surface of the substrate flat, a method of tapering the color layer pattern is also proposed. However, when the overcoat layer is not formed around the outer peripheral portion 144 of the resin BM frame 14, a steep step of the resin BM frame 14 will expose, because the color layer pattern is generally not provided on the resin BM frame 14 or the dummy pattern is arranged only on of several pixel lines. Although the partial projection with the redundant color layer pattern and the BM pattern in the display area can be made flat by the surface polishing of the color filter substrate, a steep step at the outer peripheral portion 114 of the resin BM frame 14 cannot be formed into a tapered slope. Therefore, as long as a conventional method such as tapering the color layer pattern section and a method to polish the surface of the color filter substrate is used, the planarization effect of the substrate surface is limited in the display area only. Accordingly, as to the vibration spot which occurs from the neighborhoods of the resin BM frame 14, the above-mentioned conventional method is not enough to provide an effective improvement.
In the above-mentioned patent document 2, in order to evade a problem caused by the steep step at the outer peripheral portion 144 of the resin BM frame 14, a step portion is tapered by laminating an indium tin oxide (ITO) film 21 on the surface thereof as shown in FIG. 21. However, for example, in case of the color filter substrate for the IPS mode, when the ITO film 21 is provided on the surface of the color filter substrate, a vertical electric field which disturbs a horizontal electric field for performing the IPS drive mode is generated between the color filter substrate and the TFT substrate. Therefore, the ITO film 21 with such conductivity cannot be used. In this structure, since the ITO film 21 is newly formed, the cost of the ITO material is added and thus, there is a problem that the cost reduction advantage of the non-overcoated color filter substrate disappears.
In the above-mentioned patent document 2, covering the BM pattern with the same material as an alignment film 17 is also proposed. However, when the material of the alignment film is used, it is necessary to increase the thickness of the alignment film to form the taper at the step portion, i.e., usual film thickness of tens of nanometers (nm) needs to be formed quite thick as several micrometers (μm). Accordingly, the material cost that is generally occupied in the LC panel with the alignment film of a high unit price per weight is increased further. Since the coating unevenness and residual image would influence a display quality, even if the overcoat layer is omitted, the advantage of the cost reduction disappears.
Without adding either a new process or material, a method to reduce the steep step of the resin BM layer is also proposed. For example, it is disclosed in a patent document 3 (Japanese Patent Application Laid-Open No. 2003-161826) to set the angle between the resin BM layer and a transparent substrate to be 20-55 degrees. A color filter substrate is disclosed where the angle between the resin BM layer and the colored layer is set to be 10-25 degrees. However, when the resin BM frame is tapered by selecting the material or controlling an adjustment condition, the BM pattern in the display area is also tapered simultaneously. Therefore, as shown in FIG. 10, when the resin BM layer 40 is tapered, an area 410 where the film thickness at both ends of the pattern of the BM layer 40 having width W1 becomes thin, and its linearity is degraded by the lack of optical density (OD) value and chipping at developing process. Because of this, to meet the recent demand for high-resolution display, the width of the resin BM layer is further reduced to W2 as shown in the right side of FIG. 10, and an area 420 where the film thickness occupies the large proportion of the resin BM layer, and a high-definition with the color layer pattern is difficult to achieve.